Half way up Mauna Kea on the saddle road crossing the big island of Hawaii there is a trail to the south of the road between mile 22 and 23. 9.8 kilometers down the rough Pu’uʻŌʻō trail to the right side is a Puka (Hawaiian for lava hole) about 100 feet around and 30 feet deep. This is the main entrance to the Emesine Lava Tube. This lava tube is nearly a mile long and a great hike. Bring boots, flashlights and a helmet is nice. Don’t forget sunscreen for the hike out. It’s sunny and at nearly 8,000 feet in elevation.
Info update. You now need a permit to go in the lava tube. Contact the Hawai’i Department of Land and Natural Resources at 808-587-0166.